Electrolyte and lithium ion battery

Abstract

The application provides an electrolyte and a lithium ion battery, and belongs to the technical field of secondary batteries.The electrolyte comprises a lithium salt, an organic solvent and a first additive.The first additive with a-Si-O-Si-group structure is added in the electrolyte, the-Si-O-Si-group in the first additive reacts with the silicon hydroxyl group remaining on the surface of a silicon-based negative electrode material to form a Si-O-Si bond, the first additive is attached and fixed on the surface of the silicon-based negative electrode material, and when the silicon-based negative electrode material expands in volume, the group with a large space resistance on the periphery of the Si-O-Si bond diffuses on the surface of the silicon-based negative electrode material, thereby reducing the fresh bare surface caused by the volume expansion of the silicon-based negative electrode material, inhibiting the negative influence of the volume expansion of the silicon-based negative electrode material on the lithium ion battery, and further improving the safety performance, storage performance and cycle performance of the lithium ion battery assembled with the silicon-based negative electrode.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of secondary batteries, in particular to an electrolyte and a lithium ion battery. BACKGROUND

[0002] Lithium ion batteries are widely used in electric vehicles due to their high energy density and long cycle life. With the development of the electric vehicle industry, the driving range has become an important reason for limiting the further development of lithium ion batteries.

[0003] Silicon-based negative electrode materials have extremely high theoretical specific capacity (4200 mAh / g), which can greatly improve the energy density of negative electrode sheets and are widely considered as the preferred choice for the next generation of negative electrodes.

[0004] However, although silicon-based negative electrode materials have ideal specific capacity, the volume expansion effect of silicon-based negative electrode materials during the lithium intercalation and deintercalation process of charging and discharging can easily cause the silicon-based negative electrode material to powder and separate from the negative electrode sheet, causing rapid capacity decay of the battery and poor safety and cycle performance of lithium ion batteries equipped with silicon-based negative electrodes in actual application.

[0005] Therefore, it is necessary to design an electrolyte and a lithium ion battery that can inhibit the negative effects of silicon-based negative electrode material expansion in lithium ion batteries, thereby solving the above problems. SUMMARY

[0006] In view of the above shortcomings of the prior art, the present application provides an electrolyte and a lithium ion battery for inhibiting the volume expansion of silicon-based negative electrode materials during the charging and discharging process in lithium ion batteries and reducing the negative effects of silicon-based negative electrode material volume expansion on lithium ion batteries.

[0007] To achieve the above object and other related objects, the present application provides an electrolyte, which comprises: a lithium salt, an organic solvent and a first additive.

[0008] The first additive comprises one or more of the compounds represented by structural formula (1):

[0009]

[0010] wherein R1, R2, R3, R4 and R5 are each independently selected from methyl, tert-butyl, 1,1-dimethoxyethyl ether or 1,1-diethylpropyl.

[0011] In an example of the present application, the compound represented by structural formula (1) contains an unsaturated bond.

[0012] In an example of the present application, the mass percentage content of the first additive in the electrolyte is 0.1% to 1%.

[0013] In an example of the present application, the electrolyte further comprises a second additive, the second additive comprising one or more of the compounds represented by structural formula (2):

[0014] R6-N=C=N-R7 (2)

[0016] wherein R6 and R7 are each independently selected from an alkyl group or a cycloalkyl group.

[0017] In an example of the present application, the second additive has a mass percentage content in the electrolyte of 0.1% to 0.6%.

[0018] In an example of the present application, the organic solvent comprises a cyclic carbonate and a linear carbonate.

[0019] In an example of the present application, the cyclic carbonate comprises one or both of ethylene carbonate (EC) and propylene carbonate (PC); and the linear carbonate comprises one or more of dimethyl carbonate (DMC), diethyl carbonate (DEC), or ethyl methyl carbonate (EMC).

[0020] In an example of the present application, the cyclic carbonate has a mass percentage content in the organic solvent of 10% to 40%; and the linear carbonate has a mass percentage content in the organic solvent of 10% to 80%.

[0021] The present application also provides a lithium ion battery, the lithium ion battery comprising a positive electrode sheet, a negative electrode sheet, a separator, and the electrolyte of any one of the examples described above.

[0022] The present application adds a first additive having a -Si-O-Si- group structure to the electrolyte, and utilizes the -Si-O-Si- group in the first additive to react with silicon hydroxyl groups remaining on the surface of a silicon-based negative electrode material, so that the first additive forms Si-O-Si bonds on the surface of the silicon-based negative electrode material, thereby adhering to the surface of the silicon-based negative electrode material. When the silicon-based negative electrode material expands in volume, the groups with a large steric hindrance around the Si-O-Si bonds in the first additive will diffuse away from the surface of the silicon-based negative electrode material as the silicon-based negative electrode material expands in volume, thereby reducing the fresh bare surface caused by the volume expansion of the silicon-based negative electrode material and inhibiting the negative effects of the volume expansion of the silicon-based negative electrode material on the lithium ion battery, and further improving the safety performance, storage performance, and cycle performance of the lithium ion battery equipped with the silicon-based negative electrode. DETAILED DESCRIPTION

[0023] Following are the specific examples to illustrate the embodiments of the present application, and those skilled in the art can easily understand other advantages and effects of the present application from the disclosure of the specification. The present application can also be implemented or applied by other different specific embodiments, and various modifications or changes can be made to the details in the specification based on different views and applications without departing from the spirit of the present application.

[0024] It should be noted that the following embodiments and features in the embodiments can be combined with each other without conflict. It should also be understood that the terms used in the embodiments of the present application are for the purpose of describing specific specific embodiments, and are not intended to limit the protection scope of the present application. The test methods in the following embodiments are not specified, and are generally carried out under conventional conditions or under the conditions recommended by the manufacturers.

[0025] For the sake of simplicity, only some numerical ranges are explicitly disclosed herein, and each point or individual value between the range endpoints is included in the range. Thus, each point or individual value can be combined with any other point or individual value or with other lower or upper limits to form a range not explicitly recited.

[0026] The present application provides an electrolyte and a lithium ion battery. A first additive having a-Si-O-Si- group structure is introduced into the electrolyte. The Si-O-Si bond formed by the reaction between the a-Si-O-Si- group in the first additive and the residual silicon hydroxyl on the surface of the silicon-based negative electrode material makes the first additive form a protective layer with large steric hindrance on the surface of the silicon-based negative electrode material. When the silicon-based negative electrode material expands in volume, the stress generated on the surface of the material is eliminated, and the area of the newly exposed surface of the silicon-based negative electrode material is reduced, thereby inhibiting the negative effects of the volume expansion of the silicon-based negative electrode material on the lithium ion battery and improving the safety performance, storage performance and cycle performance of the lithium ion battery.

[0027] The electrolyte provided by the present application comprises a lithium salt, an organic solvent and a first additive. The structure of the first additive is shown in formula (1):

[0028]

[0029] In structural formula (1), R1, R2, R3, R4 and R5 are each independently selected from any one of methyl, tert-butyl, 1,1-dimethoxyethyl ether group and 1,1-diethylpropyl group. That is, R1 can be methyl, tert-butyl, 1,1-dimethoxyethyl ether group or 1,1-diethylpropyl group, R2 can be methyl, tert-butyl, 1,1-dimethoxyethyl ether group or 1,1-diethylpropyl group, R3 can be methyl, tert-butyl, 1,1-dimethoxyethyl ether group or 1,1-diethylpropyl group, R4 can be methyl, tert-butyl, 1,1-dimethoxyethyl ether group or 1,1-diethylpropyl group, and R5 can be methyl, tert-butyl, 1,1-dimethoxyethyl ether group or 1,1-diethylpropyl group; wherein R1, R2, R3, R4 and R5 can all be the same, for example, R1, R2, R3, R4 and R5 are all methyl or tert-butyl; R1, R2, R3, R4 and R5 can also be partially the same, for example, R1, R2 and R3 are methyl, and R4 and R5 are 1,1-dimethoxyethyl ether group, and the like. The first additive can be one of the compounds shown in structural formula (1), or a combination of two or more compounds mixed in any ratio.

[0030] In a lithium ion battery equipped with a silicon-based negative electrode, the first additive added in the electrolyte can react with the silicon hydroxyl groups remaining on the surface of the silicon-based negative electrode material by using its own -Si-O-Si- group, and the -Si-O-Si- group of the first additive forms a Si-O-Si bond on the surface of the silicon-based negative electrode material, and then the first additive is attached and fixed to the surface of the silicon-based negative electrode material through the Si-O-Si bond. The attached first additive uses the R1, R2, R3, R4 and R5 groups on the periphery of the Si-O-Si bond to coat and protect the surface of the silicon-based negative electrode material.

[0031] When the silicon-based negative electrode material expands in volume during the charging and discharging process of the battery, the attached first additive uses the R1, R2, R3, R4 and R5 groups with larger steric hindrance on the periphery of the Si-O-Si bond to diffuse on the surface of the silicon-based negative electrode material, thereby relieving the surface stress of the silicon-based negative electrode material when it expands, reducing the fresh bare surface caused by the volume expansion of the silicon-based negative electrode material, avoiding the cracking and pulverization of the surface of the silicon-based negative electrode material, and further inhibiting the negative impact of the volume expansion of the silicon-based negative electrode material on the capacity and cycle performance of the lithium ion battery.

[0032] In some embodiments, the chemical structure of the first additive further includes an unsaturated bond, which can cause the adjacent first additive compounds attached to the surface of the silicon-based negative electrode material to polymerize, thereby enhancing the adhesion strength of the first additive on the surface of the silicon-based negative electrode material and reducing the failure of the first additive caused by the breakage of the Si-O-Si bond when the silicon-based negative electrode material expands.

[0033] In some embodiments, the first additive has a mass percentage content in the nonaqueous electrolyte of 0.1% to 1%, for example, 0.1%, 0.3%, 0.5%, 0.8%, or 1%, and the like.

[0034] In some preferred embodiments, the first additive has a mass percentage content in the nonaqueous electrolyte of 0.5% to 1%, for example, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 1%, and the like. When the mass percentage content of the first additive in the electrolyte is within the above range, the cycle performance improvement effect on the lithium ion battery equipped with a silicon-based negative electrode is optimal.

[0035] In some embodiments, the electrolyte further comprises a second additive, and the structure of the second additive is shown in structural formula (2):

[0036] R6-N=C=N-R7 (2)

[0038] In structural formula (2), R6and R7are each independently selected from an alkyl group or a cycloalkyl group. That is, R6may be an alkyl group or a cycloalkyl group, and R7may be an alkyl group or a cycloalkyl group, wherein R6and R7may be the same, for example, R6and R7are both ethyl groups, or both are cyclohexyl groups, and the like; or R6and R7may be different, for example, R6is n-butyl, and R7is cyclohexyl, and the like. The second additive can be one of the compounds shown in structural formula (2), or a combination of two or more compounds mixed in any ratio.

[0039] In the lithium ion battery equipped with a silicon-based negative electrode, the Si-O-Si bond formed by the adhesion of the first additive on the surface of the silicon-based negative electrode material is easily acidolyzed by the acidic substances (such as HF) in the electrolyte, causing the Si-O bond to break, thereby reducing the inhibitory effect of the first additive on the volume expansion of the silicon-based negative electrode material. To solve the above problem, the first additive is added in combination with the second additive in the electrolyte, and the second additive can absorb water and acidic substances in the electrolyte to reduce the influence of the formation of acidic substances (such as HF) in the electrolyte on the first additive, thereby helping to improve the inhibitory effect of the first additive on the volume expansion of the silicon-based negative electrode material, and the improvement effect on the capacity and cycle performance of the lithium ion battery.

[0040] In some embodiments, the second additive has a mass percentage content in the nonaqueous electrolyte of 0.1% to 0.6%, for example, 0.1%, 0.3%, or 0.6%, and the like. When the mass percentage content of the second additive in the electrolyte is within the above range, the protection effect on the first additive attached to the surface of the silicon-based negative electrode material is optimal, and the cycle performance improvement effect on the lithium ion battery equipped with a silicon-based negative electrode is optimal.

[0041] The organic solvent in the electrolyte can be selected from a combination of conventional solvents in the art. In the present application, the organic solvent includes cyclic carbonate and linear carbonate, wherein the mass percentage of the cyclic carbonate in the total mass of the organic solvent is 10-40%, for example, 10%, 20% or 40%, etc., and the mass percentage of the linear carbonate in the total mass of the organic solvent is 10-80%, for example, 10%, 30%, 50% or 80%, etc.

[0042] For example, the cyclic carbonate includes one or both of ethylene carbonate (EC) and propylene carbonate (PC), for example, the cyclic carbonate can be ethylene carbonate, propylene carbonate or a mixture of ethylene carbonate and propylene carbonate in any proportion.

[0043] The linear carbonate includes one or more of dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC), i.e., the linear carbonate can be any of the above-mentioned types, for example, dimethyl carbonate, diethyl carbonate or ethyl methyl carbonate; the linear carbonate can also be a combination of any two or three of the above-mentioned types, for example, a combination of dimethyl carbonate and diethyl carbonate, a combination of diethyl carbonate and ethyl methyl carbonate, or a combination of dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate. It should be noted that when the linear carbonate is a combination, the proportion of each component in the combination is not limited and can be mixed in any proportion.

[0044] The lithium salt in the electrolyte can be selected from a conventional lithium salt in the art. For example, the lithium salt includes lithium bisfluorosulfonylimide (LiFSI) and / or lithium hexafluorophosphate (LiPF6), i.e., the lithium salt can be lithium bisfluorosulfonylimide (LiFSI) alone, lithium hexafluorophosphate (LiPF6) alone or a combination of lithium bisfluorosulfonylimide (LiFSI) and lithium hexafluorophosphate (LiPF6) in any proportion.

[0045] In some embodiments, the mass percentage of the lithium salt in the electrolyte is 8-20%, further, the mass percentage of the lithium salt in the electrolyte is 10-17%, for example, 10%, 13%, 15% or 17%, etc.

[0046] The electrolyte of the present application can be prepared according to a conventional preparation method, for example, the organic solvent is mixed and stirred, then the lithium salt is added to the mixed solvent under stirring, and the stirring is continued until the lithium salt is completely dissolved, and finally the first additive and the second additive are added and stirred uniformly to obtain the electrolyte.

[0047] The application further provides a lithium ion battery, a positive electrode sheet of the lithium ion battery, a negative electrode sheet, a separator arranged between the positive electrode sheet and the negative electrode sheet, and the electrolyte described above. In the process of charging and discharging the battery, lithium ions are reversibly inserted and extracted between the positive electrode sheet and the negative electrode sheet. The separator is arranged between the positive electrode sheet and the negative electrode sheet and plays a role of isolation. The electrolyte plays a role of conducting lithium ions between the positive electrode sheet and the negative electrode sheet.

[0048] The electrolyte is the electrolyte described above. The first additive in the electrolyte is attached and fixed to the surface of the silicon-based negative electrode material through the reaction of -Si-O-Si- groups and silicon hydroxyl groups, and the group with a large steric hindrance outside the Si-O-Si bond is used to coat and protect the surface of the silicon-based negative electrode material, thereby reducing the newly exposed surface area when the silicon-based negative electrode material expands in volume, inhibiting the negative effects of the volume expansion of the silicon-based negative electrode material on the lithium ion battery, and further improving the safety performance, storage performance, and cycle performance of the lithium ion battery.

[0049] The composition and preparation method of the lithium ion battery are described below:

[0050] The positive electrode sheet of the lithium ion battery comprises a positive electrode current collector and a positive electrode active material layer arranged on at least one surface of the positive electrode current collector. The positive electrode current collector can be made of a material with good electrical conductivity and mechanical strength, such as an aluminum foil. The positive electrode current collector has two opposite surfaces in the thickness direction of itself, and the positive electrode active material layer is arranged on any one or both of the two opposite surfaces of the positive electrode current collector. The positive electrode active material layer comprises a positive electrode material, a positive electrode conductive agent, and a positive electrode binder. The specific types of the positive electrode material, the positive electrode conductive agent, and the positive electrode binder are not specifically limited here, and materials known in the art that can be used in lithium ion batteries can be selected according to actual needs.

[0051] The positive electrode material can be selected from lithium cobaltate, ternary materials, and lithium-containing phosphates. Specifically, the ternary material includes but is not limited to lithium nickel cobalt manganese oxide LiNi x Co y Mn z O2(x+y+z=1); the lithium-containing phosphate includes but is not limited to lithium manganese iron phosphate, lithium iron phosphate, lithium manganese phosphate, etc. The binder is, for example, selected from polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE), etc. The conductive agent is, for example, selected from one or a combination of two or more in any proportion of carbon black, acetylene black, graphene, carbon nanotubes, carbon nanofibers, etc.

[0052] The positive electrode sheet can be prepared according to methods known in the art. For example, the positive electrode material, the positive electrode conductive agent, and the positive electrode binder are dispersed in a solvent (e.g., N-methyl pyrrolidone, NMP for short) to form a uniform positive electrode slurry: the positive electrode slurry is coated on the positive electrode current collector, and after processes such as drying, cold pressing, etc., the positive electrode sheet is obtained. The proportions between the components in the positive electrode slurry can be set with reference to conventional proportions, which are not limited herein.

[0053] The negative electrode sheet of the lithium ion battery comprises a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. The negative electrode current collector can be made of a material with good electrical conductivity and mechanical strength, such as a copper foil. The negative electrode current collector has two opposite surfaces in the thickness direction of itself, and the negative electrode active material layer is disposed on any one or both of the two opposite surfaces of the negative electrode current collector. The negative electrode active material layer comprises a negative electrode material, a negative electrode conductive agent, a negative electrode binder, and a thickening agent. The specific types of the negative electrode material, the negative electrode conductive agent, and the negative electrode binder are not specifically limited herein, and materials known in the art that can be used in lithium ion batteries can be selected according to actual needs.

[0054] The negative electrode material is selected from one or more of artificial graphite, natural graphite, soft carbon, hard carbon, pure silicon, silicon oxide compounds, and silicon carbon compounds, and is preferably a silicon oxide compound. The negative electrode conductive agent is selected from one or a combination of two or more of carbon black, acetylene black, graphene, carbon nanotubes, carbon nanofibers, etc. in any proportion. The negative electrode binder is selected from any one of polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), and styrene butadiene rubber (SBR), or a combination of several in any proportion; and the thickening agent is selected from carboxymethyl cellulose, which can be sodium carboxymethyl cellulose (CMC-Na) or lithium carboxymethyl cellulose (CMC-Li).

[0055] The negative electrode sheet can be prepared according to methods known in the art. For example, the negative electrode material, the negative electrode binder, the thickening agent, and the negative electrode conductive agent are dispersed in deionized water to form a uniform negative electrode slurry: the negative electrode slurry is coated on the negative electrode current collector, and after processes such as drying, cold pressing, etc., the negative electrode sheet is obtained. The proportions between the components in the negative electrode slurry can be set with reference to conventional proportions, which are not limited herein.

[0056] The separator is selected from conventional types in the art, for example, a PE porous membrane is selected as the separator, the thickness of the separator is 9 to 18 μm, the air permeability is 180 s / 100 mL to 380 s / 100 mL, and the porosity is 30% to 50%.

[0057] The battery is assembled according to a conventional method, for example: after preparation, the negative electrode sheet, the separator, and the positive electrode sheet are sequentially stacked in order and are loaded into an aluminum plastic film to obtain a dry battery cell, and the dry battery cell is baked at 80 DEG C to remove water. The prepared electrolyte is injected into the dry battery cell to obtain a finished lithium ion battery.

[0058] The technical solutions of the present application are described in detail below through several specific examples and comparative examples. Unless otherwise specified, the raw materials and reagents used in the following examples are commercially available or can be prepared by conventional methods in the art, and the instruments used in the examples are commercially available.

[0059] The material components of examples 1 to 7 and comparative example 1 are shown in Table 1.

[0060] Example 1

[0061] The present example provides an electrolyte, which comprises 2% of lithium hexafluorophosphate (LiPF6) by mass percentage, 13% of lithium bisfluorosulfonylimide (LiFSI) by mass percentage, and 0.1% of a first additive by mass percentage, the first additive being a compound represented by structural formula (1), the R1, R2, R3, R4, and R5 groups in the first additive being methyl groups; and the remaining part of the electrolyte being an organic solvent, the organic solvent comprising 30% of ethylene carbonate (EC), 50% of methyl ethyl carbonate (EMC), and 20% of diethyl carbonate (DEC).

[0062] The preparation method of the electrolyte comprises the following steps: uniformly mixing ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) in a mass ratio of 30:50:20 to obtain an organic solvent; adding 2wt% of lithium hexafluorophosphate and 13wt% of lithium bisfluorosulfonylimide into the organic solvent and uniformly mixing; then adding 0.1wt% of the compound represented by structural formula (1) as a first additive, and uniformly mixing to obtain the electrolyte.

[0063] Example 2

[0064] The present example prepares an electrolyte of the same system as example 1, and the difference between the present example and example 1 is that the amount of the first additive in the prepared electrolyte is 0.5% by mass percentage of the electrolyte.

[0065] Example 3

[0066] The present example prepares an electrolyte of the same system as example 1, and the difference between the present example and example 1 is that the amount of the first additive in the prepared electrolyte is 1% by mass percentage of the electrolyte.

[0067] Example 4

[0068] The embodiment differs from the embodiment 1 in that the first additive is replaced by a second additive in the prepared electrolyte, the second additive is a compound shown in the structural formula (2), and the R6 and R7 groups in the second additive are cyclohexyl groups.

[0069] Embodiment 5

[0070] The embodiment prepares the electrolyte of the same system as the embodiment 4, and the embodiment differs from the embodiment 4 in that the amount of the second additive in the prepared electrolyte is 0.3% by mass percentage of the electrolyte.

[0071] Embodiment 6

[0072] The embodiment prepares the electrolyte of the same system as the embodiment 4, and the embodiment differs from the embodiment 4 in that the amount of the second additive in the prepared electrolyte is 0.6% by mass percentage of the electrolyte.

[0073] Embodiment 7

[0074] The embodiment differs from the embodiment 1 in that the first additive with a mass percentage of 0.5% and the second additive with a mass percentage of 0.3% are added in the prepared electrolyte, the first additive is a compound shown in the structural formula (1), the R1, R2, R3, R4 and R5 groups in the first additive are methyl groups, the second additive is a compound shown in the structural formula (2), and the R6 and R7 groups in the second additive are cyclohexyl groups.

[0075] Comparative Example 1

[0076] The comparative example 1 provides an electrolyte without adding the first additive and the second additive, the electrolyte includes lithium hexafluorophosphate (LiPF6) with a mass percentage of 2% and lithium bisfluorosulfonylimide (LiFSI) with a mass percentage of 13%, and the rest is an organic solute, the organic solute includes 30% ethylene carbonate (EC), 50% methyl ethyl carbonate (EMC) and 20% diethyl carbonate (DEC).

[0077] The electrolyte preparation method includes the following steps: uniformly mixing ethylene carbonate (EC), methyl ethyl carbonate (EMC) and diethyl carbonate (DEC) in a mass ratio of 30:50:20 to prepare an organic solvent; and uniformly mixing 2wt% lithium hexafluorophosphate and 13wt% lithium bisfluorosulfonylimide into the organic solvent to obtain the electrolyte.

[0078] Table 1: Component proportioning of the electrolytes prepared in the embodiments 1 to 7 and the comparative example 1

[0079]

[0080] The electrolyte prepared in Examples 1 to 7 and Comparative Example 1 was used in lithium ion batteries respectively to verify the efficacy of the present application. The preparation process of the lithium ion battery is as follows:

[0081] (1) Positive electrode sheet: LiNi 0.8 Co 0.1 Mn 0.1 O2, conductive agent acetylene black, binder polyvinylidene fluoride were dissolved in N-methyl pyrrolidone (NMP) in a mass ratio of 95:3:2, homogenized, coated on an aluminum foil, and then dried, cold-pressed, and cut to obtain a positive electrode sheet.

[0082] (2) Negative electrode sheet: After the negative electrode active material silicon monoxide, conductive agent acetylene black, and binder sodium carboxymethyl cellulose were thoroughly stirred and mixed uniformly in a deionized water solvent system in a mass ratio of 96:2:2, they were coated on a copper foil, dried, cold-pressed, and cut to obtain a negative electrode sheet.

[0083] (3) Separator: A polyethylene film with a thickness of 9 pm was used as the base film, and a nanometer alumina coating with a thickness of 3 pm was coated on the base film to obtain a separator.

[0084] (4) Battery assembly: The prepared positive electrode sheet, separator, and negative electrode sheet were stacked in sequence, with the separator between the positive and negative electrode sheets to act as a barrier, and the stack was obtained as a bare cell; the bare cell was placed in an aluminum plastic film, then baked at 80°C to remove water, and then the corresponding electrolyte was injected and sealed. After that, the finished soft-packaged lithium ion secondary battery was obtained after the processes of standing, hot and cold pressing, formation, clamping, and capacity distribution.

[0085] The lithium ion batteries assembled with the electrolytes of Examples 1 to 7 and Comparative Example 1 were tested for performance, and the test results are shown in Table 2, and the test method is as follows:

[0086] At 25°C, the lithium ion battery was charged at 1 / 3C constant current to 4.35V, and then charged at constant voltage until the current was 0.05C. Then the battery was discharged at 1 / 3C constant current to 2.5V. Record the capacity C0, repeat the above charging and discharging steps 1000 times, and record the discharge capacity C1 of 1000 cycles. The capacity retention rate of the battery is C1 / C0 x 100%.

[0087] Table 2: Battery performance of lithium ion batteries prepared in Examples 1 to 7 and Comparative Example 1

[0088]

[0089] From the test results of Comparative Examples 1 to 3 and Comparative Example 1, it can be seen that the addition of the first additive to the electrolyte can significantly improve the cycle performance of the lithium ion battery. Among them, the first additive can be attached and fixed on the surface of the silicon-based negative electrode material through the Si-O-Si bond formed by polymerization, and the surface of the silicon-based negative electrode material can be protected by using the R1, R2, R3, R4 and R5 groups with large steric hindrance around the Si-O-Si bond, eliminating the surface cracks caused by volume expansion of the silicon-based negative electrode material during charging and discharging, thereby effectively inhibiting the negative effects of volume expansion of the silicon-based negative electrode material during charging and discharging, and the cycle capacity retention rate of the lithium ion battery is increased from 73.1% before adding the first additive to more than 79.8% after adding the first additive.

[0090] And from the test results of Examples 1 to 3, it can be seen that within the appropriate dosage range (0.1% to 1% of the mass percentage of the electrolyte), as the dosage of the first additive increases, the improvement of the cycle capacity retention rate of the lithium ion battery also gradually increases, for example, from 79.8% when 0.1% of the first additive is added to 84.4% when 1% of the first additive is added.

[0091] From the test results of Comparative Examples 4 to 6 and Comparative Example 1, it can be seen that the addition of the second additive to the electrolyte alone has a certain improvement effect on the cycle performance of the lithium ion battery, but overall, the improvement effect of the second additive alone on the cycle performance of the lithium ion battery is not obvious, and the cycle capacity retention rate of the lithium ion battery is only increased from 73.1% before adding the second additive to about 75% after adding the second additive. It can be seen that the improvement of the cycle capacity retention rate of the lithium ion battery by adding the second additive is only about 1% to 3%.

[0092] And from the test results of Examples 4 to 6, it can be seen that within the appropriate dosage range (0.1% to 0.6% of the mass percentage of the electrolyte), as the dosage of the second additive increases from 0.1wt%, the cycle capacity retention rate of the lithium ion battery also gradually increases; until the dosage of the second additive reaches 0.3wt%, the cycle capacity retention rate of the lithium ion battery reaches the highest; and then when the dosage of the second additive continues to increase, the cycle capacity retention rate of the lithium ion battery will decrease instead. It can be seen that when the mass percentage of the second additive in the electrolyte is 0.3%, the improvement effect on the cycle performance of the lithium ion battery is best.

[0093] From the test results of Comparative Example 2 and Example 7, it can be seen that, on the basis of adding the first additive, the second additive is added to further improve the cycle performance of the lithium ion battery. Since the second additive can absorb water and acidic substances in the electrolyte, it can protect the first additive attached to the surface of the silicon-based negative electrode material, thereby helping to improve the inhibition of the first additive to the volume expansion of the silicon-based negative electrode material, and further improving the cycle performance of the lithium ion battery, for example, the cycle capacity retention rate of the lithium ion battery is improved from 83.2% when only 0.5% of the first additive is added to 89.4% when 0.5% of the first additive and 0.3% of the second additive are added.

[0094] The first additive with -Si-O-Si- group structure is added to the electrolyte, and the -Si-O-Si- group in the first additive reacts with the silicon hydroxyl group remaining on the surface of the silicon-based negative electrode material to form a Si-O-Si bond, so that the first additive is attached and fixed on the surface of the silicon-based negative electrode material, and when the silicon-based negative electrode material expands in volume, the group with a large steric hindrance on the periphery of the Si-O-Si bond diffuses on the surface of the silicon-based negative electrode material, thereby reducing the fresh bare surface caused by the volume expansion of the silicon-based negative electrode material, and inhibiting the negative effects of the volume expansion of the silicon-based negative electrode material on the lithium ion battery.

[0095] At the same time, the second additive added in the electrolyte in combination with the first additive can effectively absorb water and acidic substances formed in the electrolyte, protect the first additive attached to the surface of the silicon-based negative electrode material, and avoid the inactivation of the attached first additive by acid, thereby helping to improve the inhibition of the first additive to the volume expansion of the silicon-based negative electrode material, and further improving the cycle performance of the lithium ion battery.

[0096] The above examples are only illustrative of the principles and effects of the present application, and are not intended to limit the present application. Any person skilled in the art can modify or change the above examples without departing from the spirit and scope of the present application. Therefore, all equivalent modifications or changes made by those skilled in the art without departing from the spirit and technical idea disclosed by the present application should be covered by the claims of the present application.

Claims

1. An electrolyte, characterized by, The electrolyte comprises: a lithium salt, an organic solvent, a first additive and a second additive, the first additive comprising one or more of compounds represented by structural formula (1): (1) wherein R1, R2, R3, R4 and R5 are each independently selected from methyl, tert-butyl, 1,1-dimethoxyethyl ether or 1,1-diethylpropyl; the second additive comprising one or more of compounds represented by structural formula (2): (2) wherein R6 and R7 are each independently selected from alkyl.

2. The electrolyte of claim 1, wherein The mass percentage content of the first additive in the electrolyte is 0.1% to 1%.

3. The electrolyte of claim 1, wherein The mass percentage content of the second additive in the electrolyte is 0.1% to 0.6%.

4. The electrolyte of claim 1, wherein The organic solvent comprises a cyclic carbonate and a linear carbonate.

5. The electrolyte of claim 4, wherein The cyclic carbonate comprises one or both of ethylene carbonate (EC) and propylene carbonate (PC); the linear carbonate comprises one or more of dimethyl carbonate (DMC), diethyl carbonate (DEC) or ethyl methyl carbonate (EMC).

6. The electrolyte of claim 4, wherein The mass percentage content of the cyclic carbonate in the organic solvent is 10% to 40%; the mass percentage content of the linear carbonate in the organic solvent is 10% to 80%.

7. The electrolyte of claim 1, wherein The lithium salt comprises lithium bisfluorosulfonylimide and / or lithium hexafluorophosphate, the mass percentage content of the lithium salt in the electrolyte being 8% to 20%.

8. A lithium-ion battery, characterized by, The electrolyte comprises a positive electrode sheet, a negative electrode sheet, a separator and any one of the electrolytes of claims 1 to 7.

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